The concepts involved in Newton's three laws are so basic to understanding the behavior of objects in the universe that they appear in almost all branches of physics. In order for you to understand the rest of the material in this text you should make sure that you understand Newton's three laws. Newton's Second Law refers to acceleration, so it might help your understanding of this chapter if you review Chapter 3 either before you begin your study of this chapter or while you are studying this chapter. It will be difficult for you to fully understand how to use Newton's Second Law to calculate an acceleration if you do not understand what an acceleration really is. The basic expression for Newton's Second Law is F = m a where F is the net force acting on the object, m is the mass, and a is the acceleration. (In this study guide vector quantities are identified by bold print.)

Newton's First Law tells us that an object will stay at rest or will continue in straight line motion at a constant velocity unless it experiences an unbalanced force. If there is no net force, there is no change in velocity.

Newton's Second Law tells us how much of a change in velocity is produced when an object experiences a particular net force. The larger the force on a given object the greater is the acceleration, hence the greater the change in velocity. (Remember from Chapter 3 that acceleration is the rate of change of velocity). In the equation expressing Newton's Second Law, F = m a, mass appears as the proportionality constant relating the acceleration to the force. Newton's Second Law is a vector equation in which the direction of the acceleration is the same as the direction of the net force.

A force of magnitude 1 Newton produces an acceleration of 1 m/s² on an object of mass 1 kilogram. We will learn in a later chapter that 1 liter (1000 cm³) of water has a mass of 1 kilogram.

Newton's Third Law states that forces always occur in pairs with an action force on one object always accompanied by an equal force called the reaction force acting in the opposite direction on the second object.

Solution of problems in physics that involve forces is often made easier by using a drawing representing the object and all the forces acting upon it. This type of drawing is called a Free Body Diagram. The purpose for drawing a free body diagram is to have a picture of the object, often just a simple line drawing, with the external forces represented as vectors. The object is drawn in isolation from all its surroundings. This means that any contacts that the object has with other objects such as with the floor must be represented as forces. It is often quite easy to determine the net force acting on an object once the free body diagram has been drawn. Time taken to identify the forces and to properly draw a free body diagram will be rewarded in the way in which it will simplify the solution of a problem.